Electromagnetic wave filters are used in a wide variety of applications. Filters are used to eliminate some frequencies (referred to as the rejection band) from a signal. The frequencies which are not eliminated are referred to as the passband. When the frequencies the filter acts upon are in the microwave portion of the electromagnetic spectrum, the filter can be referred to as a microwave filter. The edge, or edges, of the rejection band are typically referred to as a cutoff. Considerations when selecting a filter include sharpness of the cutoff (also referred to in the field as skirt steepness and roll-off) and Q factor. The sharpness of the cutoff can affect the narrowness of a passband, for instance. Tunability of the cutoff frequency is required in some applications, such as in reconfigurable communication systems. It is known to provide tunability of such filters in a mechanical manner using means such as a motor mechanism or other mechanical structure. However, this technique has disadvantages such as size, cost, performance limitations, etc.
High quality factor (High-Q) tunable filters are needed in both wireless and satellite systems. Tunability and re-configurability in these systems can offer great flexibility and economic benefits. For example, one of the challenges facing wireless service providers is the real-estate cost during base station installation in urban areas. The inclusion of tunable filters that integrate many functions, such as multi-standard and multiband filters, into one site provides an economic incentive for the wireless service provider to incorporate tunable filters into the base station instead of fixed-frequency filters. In satellite systems, use of tunable filters can significantly reduce the size and mass of the payload due to multimode and multifunctional operation and re configurability such filters offer. This reduction in mass and size has an economic impact on the cost of satellite systems as launch costs depend on satellite weight.
It is typically desirable that tunable filters exhibit a high loaded-Q value. The loaded-Q value of the tunable filter is determined by the Q value of the filter structure itself and by the insertion loss of the tuning element in use. Currently available systems employ various types of tuning elements. Some are based on semiconductors, ferroelectric materials, ferromagnetic materials, and/or mechanical systems, for instance. Integration of tuning elements within filters can increase insertion loss and result in a lower loaded-Q value.